Vaporizing heater for vacuum deposition and method of employing the same



Nov. 29, 1960 P. ALEXANDER 2,962,538

VAPORIZING HEATER FOR VACUUM DEPOSITION AND METHOD 0F 'EMPLOYING THESAME Filed Jan. 30, 1958 2 l0 F I, ,l I .I Il, Il IIIlI "III'Q I, III Il"Il,

FIG. I.

FIG. 2.

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' ATTORNEYS United States Patent Ofl'ice 2,962,538 Patented Nov. 29,1960 VAPORIZING HEATER FOR VACUUM DEPOSI- TION AND METHOD F EMPLOYINGTHE SAME Paul Alexander, Princeton, NJ., assigner to Continental CanCompany, Inc., New York, N.Y., a corporation of New York Filed Jan. 30,1958, Ser. No. 712,099

Claims. (Cl. 13-25) This invention relates to the vaporiz-ation of highboiling metals and deposition of the metal vapor upon a substrate, andis particularly concerned with a vaporizing element or heater for suchpurpose, and methods of making and using the same.

It has been proposed in the art to employ a body of electricallyconductive material, such as carbon or metals or metal compounds, whichis refractory and remains solid at the temperature of operation, to heatthe body by its electrical resistance to current flow through it, and tobring the deposit metal to the body so that the deposit metal is meltedand spreads out on the body; with employment of a body temperature atwhich the metal will evaporate. Such work is customarily done in vacuumto secure the uninhibited spreading of the metal vapor, to reduce thetemperature for vaporization, and to protect the deposit metal againstoxidation.

For regular vaporization of the molten metal, it is desirable that suchmetal be present on the Surface of the heater as a continuouslyreplenished thin film, and that it be protected against contamination bymaterials such as the carbide formed when molten aluminum acts uponcarbon. Hence carbon is not a satisfactory material for contact with themolten deposit materials; and surface coatings of protective materialhave been employed, or a non-carbon element is provided. For example,materials such as tungsten metal, tungsten carbide, and tungstensilicide, zirconium metal and zirconium carbide, titanium metal andtitanium carbide have been employed as protective coatings on carbon asset out in my prior patents and patent applications: these give a farlonger operational life than unprotected carbon. lt has also beenproposed to employ refractory borides and nitrides: and these have beenfound satisfactory in many cases. However, such bodies, or bodies havingsuch surfaces, change in their behavior during operation. At first, thedeposit metal does not form an extended regular film upon the bodies,but later the spreading occurs more regularly: so that there is aninitial operational stage of conditioning the element, and if one ofseveral vaporizer elements has to be replaced, t must be conditionedregardless of the operational status of the other elements. This initialbehavior is more pronounced when the surface, e.g. of a titanium borideelement, is smooth. Titanium boride is highly resistant to moltenaluminum; and when a smooth freshly prepared surface thereof receivesmolten aluminum, it is not readily wetted even at temperatures as highas 1500 degrees C. When an element having an effective surface of about1400 square millimeters has been operated for about an hour, and about150 to 200 grams of aluminum evaporated from it, the heater receives thealuminum more smoothly and a thin l'ilm is formed because the moltenaluminum will then spread over the surface. This preconditioning effectappears permanent; as the element may be cooled and reheated as anincident of a continued operation, without requiring a furtherconditioning at each re-heating.

The failure to produce a regular film by the spreading of the depositmetal is disadvantageous, because the molten metal may collect in beadsor pools which are deep and in which a high temperature gradient ordifferential exists between the bottom, which is at the temperature ofthe vaporizer element, and the top, which is being cooled by the loss ofthe latent heat of evaporation of the metal. When this differential ishigh, so that the vapor pressure of the aluminum adjacent the element isgreater than the hydrostatic pressure of the metal above it, bubbles ofvapor form at the bottom of the pool or bead and rise rapidly to thesurface, often exploding and projecting globules of metal instead ofexclusively issuing as a vapor of molecular particles. If such globulesreach the substrate to be coated, they damage the coating and oftenpierce holes in a plastic film or paper present as the substrate. Inaddition, the pre-conditioning does not proceed regularly, as thepresence of molten aluminum in the illus trative case is necessary incontact with the area: hence the preconditioning effect first occurs atthe area which is receiving the replenishment supply, and then spreadsover the rest of surface. Therewith etching or corrosion during the lifeof the element is irregular and a substantially uniform film thicknessis not maintained.

Such pre-forming or pre-conditioning of a vaporizer element may beperformed in the absence of a substrate. When a coating operation isbeing started with the vaporizer elements all fresh, a substrate may beintroduced; but care must be taken to feed the deposit metal veryslowly, so that the molten metal does not exceed one millimeter in depthat any point and to keep the temperature of the heater correspondinglylow: therewith the rate of evaporation is very low, and the rate of feedof a continuous substrate must be reduced until the specified thicknessof deposit is effected because unless this is done, the substrate isexcessively heated and may be damaged. and the production isuneconomical.

Wettability of the vaporizer element surface by the molten depositmaterial may be effected, as stated above, by employing a suitablerefractory substance as a coating or covering layer; but the property ofWettability appears connected with the property of solubility: that is,when the coating material is relatively soluble, the molten materialspreads readily; whereas if the coating material has a vanishingly smallsolubility, the molten material fails lto spread on a smooth surfacethereof. For example, tungsten metal can be employed, but in course oftime, it is dissolved in molten aluminum and the resultant liquiddemands a higher operating temperature. As the molten deposit metalevaporates from the mixture, the tungsten remains behind at thevaporization surface, and at the margins it is deposited upon the bodyof the element beyond the area orginally occupied, so that the coatingappears to creep out over the vaporizer surface and toward its edges.Since the precipitated metal is wettable, it thus acts as a wick to drawthe molten aluminum toward and over the edges of the vaporizer. Thevaporized metal establishes a zone over the vaporization area, with theparticles tending to move in directions normal to the surface unit atwhich the evaporation occurred. Thus, aluminum evaporated from a freshheater element can be directed so that most of it encounters thesubstrate; but after the creepage has occurred, aluminum beingevaporated from upright edges will not be directed toward an overlyingsubstrate, and efiiciency is lost.

It has been found that vaporizer elements may be made of substances,which are refractory and otherwise satisfactory for employment exceptfor not being wetted by the molten deposit material when the same isfirst applied thereto, by providingy the area from which the metal is tobe evaporated with a number of narrow and shallow grooves. These canform parallel lines placed longitudinally in the top surface or I'theycan cross each other forming a diamond pattern on the surface. Themolten metal spreads over the surface along these lines apparently incapillary fashion due to the action of .physical forces at the interfaceregions, and forms a thin -lm'over all the intended operating surfacewithout con- Vin which:

Fig. l is a top view of such an element;

Fig. 2 is a cross-section substantially on line 2-2 of Fig. l;

Figs. 3 and 4 are top views of modified structures;

Fig. 5 is an enlarged section on line 5-5 of Fig. 4.

The vaporizer elements shown in Figs. l to 5 may be made of refractorymaterials such as the carbides, borides, nitrides and silicides of thetransitional metals belonging to groups IV, V and VI of the periodicsystem of the elements. Titanium diboride, TiB2, is a specific example.Such elements may be made by molding under high pressure; and the bodiesthen subjected to a pre-sintering heating. At this stage the bodies aremachinable and can be given the desired final shapes.

In the structure of Fig. l, the general body has a depression 11 in itsupper surface. The bottom or floor of the depression has multiple spacedgrooves 12 therein; which are exaggerated in size in the drawing, forclearness.

IllustrativelyY the body l0 may have a length of 150 millimeters, awidth of 18 millimeters, and a thickness of 7 millimeters, with thedepression 11 having its oor one millimeter below the general top planeof the element. The depression thus has a depth of one millimeter with awidth of 14 millimeters, and a length of 100 millimeters, the ends beingsemi-circles of 7 millimeter radius. The grooves 12 in this illustrativeform are for example 0.010 to 0.050 inch wide and 0.005 to 0.060 inchdeep, with flat crests therebetween providing the floor level and havingwidths of 0.020 to 0.100 inch, for example each groove can be about0.020 inch wide, 0.010 inch wide and with at crests 0.0625 inch Wide. Insuch a structure, the depression 11 has a iioor at a depth less thanonefourth, being about one-seventh in the illustrated form, of thevertical thickness of the body mass beneath it; and the minimumhorizontal dimension of the upstanding portions around the depression 11is twice the depth of the depression itself.

A number of the grooves should be present to facilitate distribution ofthe metal to be deposited over the length and width of the depressionfloor, from the point at which it is momentarily being melted during itsdelivery into the depression; noting that the preferred practice is toemploy a wire which is being continually advanced with its end broughtinto the depression and there melted, the end of the wire beingtraversed back and forth along the length of the depression 11.

The grooves need not be in the nature of hairline cracks; and it ispreferred to have each groove not less than 0.010 inch wide and not lessthan 0.005 inch deep. Therewith, each groove should coordinate in widthwith the intervening and marginal crest regions; and it is preferred inpractice to have the width of each groove not greater than one-tenth ofthe width of the depression oor, and its depth not greater thanone-tenth of the vertical distance from the depression oor to the bottomsurface of the element body. Therewith, the spacing of the grooves, fromcenter to center in Figs. l and 2, should not be less than one-fifteenthnor more than onefourth of the width of the depression, the specificnumber of grooves being determined by such selected spacing and byhaving the individual grooves within the stated ranges of related widthand depth. The grooves can terminate within the top area, e.g. the oorin the illus-x trative showing.

The grooves may be cut in the body while it is in the pre-sinteredstate; and the final sintering then accomplished. It is also feasible toprovide the distributing grooves during the initial pressure molding byproviding a forming punch which is a matrix for the intended topconformation of the vaporizer body, e.g. by having ribs on the punch forforming the grooves in the mass being pressed. The mass is then heatedto produce the final sintered structure.

When a fresh element of this type is heated in a vacuum chamber, andaluminum Wire fed so its end maintains contact with the vaporizingelement, the aluminum melts and spreads readily along the surfaceWithout forming local comparatively deep pools or beads; and the elementmay be operated from the beginning, without requiring pre-conditioning,under operating conditions and without exhibition of spitting orsputtering of globules. A further advantage of such an element is thatit maintains during its entire operational life a better and more evenspreading of the aluminum iilm than an element without grooves. Thisfact again results in a longer useful life of the element, because thecorrosion is more evenly distributed over the whole vaporizing surface.

In Fig. 1, the grooves 12 are shown as parallel and extending along thelength of the vaporizing surface of the heater element.

In Fig. 3, the grooves 12a are shown as parallel lines in two groupswhich intersect in a diamond pattern, the grooves being exaggerated inwidth and spacing, for clearness.

In Fig. 4, the grooves 12b in each of two groups are shown closertogether than in Fig. 1, and at a smaller angle between the groups thanin Fig. 2, so that the bottom of the depression 11 is forming of anumber of long needlealike elevations having sharp upper edges, as shownin Fig. 5, adjacent the intersections. In Fig. 4, the grooves are shownas single lines.

The refractory carbide, boride, nitride and silicide material are oftenreferred to as hard metals, and have been used as abrasives and cuttingtools because of their hardness. k v

In each example, a thin film of volatilizable metal is being exhibitedin a shallow trough above a thick body of resistively heated material.In heating by electrical resistance, the same amperage is present ateach successive transverse cross-section along the length of the body.Therefore, assuming homogeneity, a portion with a large cross-sectionwill have a lesser current intensity of arnperes per unit area, ascompared with a section which is smaller and thus has 'more amperes perunit area. This same principle applies to the individual portions of across-section. When a large amperage is iiowing, the wattage effect isgreater, and there is greater heating and thus a higher temperature ispresent.

The illustrative practices are not restrictive, and the invention may beutilized in many ways within the scope of the appended claims.

What is claimed is:

l. A vaporizing element for the deposition of metals, composed of arefractory material selected from the class consisting of the carbides,borides, nitrides and silicides of the transitional metals belonging togroups IV, V, and

VI of the periodic system and having a body with a top depression, thedepth of the body beneath the oor of the depression being at least fourtimes the depth of the depression, the walls bounding the depressionhaving a greater horizontal dimension than the depth of the depression.

2. A vaporizing element as in claim 1, in which the body is composed ofa sntered mass of a refractory hard metal material, and in which thedepression has a depth of about one millimeter and the depth of the bodybeneath the oor of the depression is about seven times the depth of thedepression.

3. A vaporizing element as in claim 1, n which the oor of the depressionhas a roughened surface.

4. A vaporizing element as in claim 1, in which the Hoor of thedepression has grooves therein.

5. A vaporizing element as in claim 4, in which the grooves are inparallel sets intersecting one another.

6. A vaporizing element in claim 4, in which the grooves are defined bylong needle-like elevations.

7. A vaporizing element for the deposition of metals, composed of arefractory material resistant to wetting by the molten metal to bevaporized, and having a body with a top area having a plurality ofgrooves with widths of 0.010 to 0.050 inch and depths of 0.005 to 0.060inch, with intervening crests located substantially in a horizontalplane, said grooves terminating within the said area.

8. A vaporizing element as in claim 7, in which the grooves are paralleland are spaced 0.020 to 0.100 inch apart.

9. A vaporizing element as in claim 7, in which the top of the elementhas a fiat area and the grooves are about 0.02.0 inch wide, 0.010 inchdeep, and are spaced about 0.0625 inch; each groove being no wider thanone-tenth of the width of the at top area and no deeper than onetenth ofthe vertical distance from the said top area to the bottom surface ofthe element body, and the grooves being spaced from center to center adistance between one-fifteenth and one-fourth of the width of said toparea.

10. A vaporizing element for the deposition of metals, composed of arefractory material resistant to wetting by the molten metal to bedeposited, and having a body of generally rectangular section at itsends for the conduction of electric current thereto, and a topdepression intermediate the ends, said depression having a depth ofabout one millimeter and having in its oor a plurality of grooves withWidths of 0.010 to 0.050 inch and depths of 0.005 to 0.060 inch, thedepth of the body below the floor of the depression being at least fourtimes the depth of the depression :and at least four times the depth ofthe grooves.

References Cited in the file of this patent UNITED STATES PATENTS2,336,138 Van Hoorn et al Dec. 7, 1943 2,557,530 Bancroft June 19, 19512,615,060 Marinace et al. Oct. 21, 1952 2,693,521 Alexander Nov. 2, 19542,772,318 Holland Nov. 26, 1956 FOREIGN PATENTS 17,852 Great Britain Y Y-1- of 1901

1. A VAPORIZING ELEMENT FOR THE DEPOSITION OF METALS, COMPOSED OF AREFRACTORY MATERIAL SELECTED FROM THE CLASS CONSISTING OF THE CARBIDES,BORIDES, NITRIDES AND SILICIDES OF THE TRANSITIONAL METALS BELONGING TOGROUPS IV, V, AND VI OF THE PERIODIC SYSTEM AND HAVING A BODY WITH A TOPDEPRESSION, THE DEPTH OF THE BODY BENEATH THE FLOOR OF THE DEPRESSIONBEING AT LEAST FOUR TIMES THE DEPTH OF THE DEPRESSION, THE WALLSBOUNDING THE DEPRESSION HAVING A GREATER HORIZONTAL DIMENSION THAN THEDEPTH OF THE DEPRESSION.